1. Field of the Invention
The present invention relates to a wet type electrophotographic color printer, and more particularly to a detecting apparatus for a wet type electrophotographic color printer for detecting various kinds of transfer jams occurring in a transfer roll, and accordingly stopping the operation of the printer.
2. Description of the Prior Art
Generally, a wet type electrophotographic color printer prints a desired image by forming the electrostatic latent image through the processes of: radiating a laser beam to a photosensitive medium such as a photosensitive belt; developing the electrostatic latent image formed on the photosensitive medium with a developing solution composed of a solid toner of a certain color and a liquid carrier as a solvent; and transferring the developed form to printed matter.
Such a wet type electrophotographic color printer includes a photosensitive material in the form of a belt reeled to travel on rollers which are disposed within a printer body. Around the photosensitive material, a charged unit, an exposure unit, a developing unit, a drying unit, and a transferring/fixing unit, etc. are disposed.
The electrostatic latent image is formed on the photosensitive medium, to be developed by the developing unit. The developing unit develops the electrostatic latent image on the surface of the photosensitive medium by jetting the developing liquid, and exclusively retaining the toner of the developing liquid at the area of electrostatic latent image. The drying unit eliminates residue of the carrier left from the operation of the developing unit, to an extent acceptable for the transferring operation. The transferring/fixing unit transfers the image formed on the photosensitive medium onto the printed matter.
The printed matter is stored in a plurality of cassettes in accordance with the respective sizes and kinds thereof, and is fed to the transferring/fixing unit along a feeding path. After the image is printed on the printed matter which is passed through the transferring/fixing unit, the printed matter is stored in a distributing tray.
Here, while the printed matter is passed between the transfer and fuser rolls, various kinds of transfer jams may occur, that is, the printed matter may be wrapped around the transfer and fuser rolls due to various causes such as peel force increase in the transfer and fuser rolls, or a stain on the transfer and fuser rolls, etc.
When the above-described transfer jams occur, the printing is not properly performed due to the crumpled exit of the printed matter. Even worse, if the printer keeps operating in the state where the printed matter is wrapped around the transfer and fuser rolls, a serious error will result in the photosensitive material, the transfer roll, the fuser roll, and the photosensitive material driving section. In order to prevent such an error, the wet type electrophotographic color printer generally has a transfer jam detecting apparatus for stopping the operation of the printer upon detecting the presence of the transfer jam.
A conventional transfer jam detecting apparatus includes a first detecting lever, which can rotate in a forward or reverse direction, disposed on the printed matter conveying path after the transfer and fuser rolls, a second detecting lever which rotates in accordance with the rotation of the first detecting lever, and a sensor which is turned on/off in accordance with the rotation of the second detecting lever.
Here, the first detecting lever is rotated clockwise on a pin by making contact with the front end of the printed matter, thereby rotating clockwise the second detecting lever on a pin. After the clockwise rotation, the first and second detecting levers are returned to their initial position by their respective weights. The sensor is turned on by the rotation of the second detecting lever, thereby sensing the normal exit of the printed matter. If the sensor is not turned on by a certain period of time after the entrance of the printed matter into the transfer roll, it is sensed that a jam has occurred, and a signal is transmitted to a control section for stopping the operation. Accordingly, the transfer jam detecting apparatus also has a sensor for sensing the point where the printed matter enters the transfer roll.
The conventional transfer jam detecting apparatus described above has the advantage of easily detecting the presence of certain kinds of jams by the sensor which senses if the printed matter reaches the desired position in a given time after the printed matter enters into the transfer roll. The conventional transfer jam detecting apparatus, however, has a shortcoming in not detecting the presence of some kinds of jams such as wrap jams, in which the middle portion of the printed matter is wrapped in the transfer roll or the fuser roll after the normal exit of the printed matter. When a wrap jams occur, the printed matter may be wrapped further into the cleaning roll, resulting in a serious error in the related parts of the printer. Further, the un-jamming process is complex.
Also, with the conventional transfer jam detecting apparatus, the printed matter is often wrapped into the cleaning roll, since there is a small difference between the distance (hereinafter called `normal distance`) that the printed matter travels during normal operation, compared to the distance (hereinafter briefly called `abnormal distance`) that the printed matter travels when wrapped in the transfer roll before being wrapped in the cleaning roll. The normal distance is expressed by the distance of the vertical line from the nip of the transfer roll to the first detecting lever, which is approximately 35 mm in the conventional apparatus, while the abnormal distance is expressed by the distance of a circular arc of the transfer roll starting from the nip of the transfer roll to the point of the transfer roll making contact with the cleaning roll, which is approximately 38 mm in the conventional apparatus. Accordingly, the difference between the normal distance (35 mm) and the abnormal distance (38 mm) is approximately 3 mm, which is quite small. Since there is a small difference between the normal and abnormal distances, it often occurs that the printed matter is allowed to advance into the cleaning roll even after detecting abnormality of the exit of the printed matter. Once the printed matter is wrapped in the cleaning roll, the user can not remove the printed matter easily.
Further, the parts of the conventional transfer jam detecting apparatus are disposed on different units, that is, the first detecting lever is attached to frame of the transferring/fixing unit, while the second detecting lever and the sensor are attached to a frame of a distributing unit. Such a dual operational structure causes difficulty in maintaining accuracy, and unreliability in operation of the printer such as unsmooth return of the detecting lever, etc.
Further, in the conventional transfer jam detecting apparatus, the sensor is disposed near the center of the fuser roll which is very hot. Accordingly, a high temperature sensor is required, and the manufacturing cost increases.